This invention relates to a sensor for the detection of smoke and, particularly, a sensor which is treated with a chemical coating for reacting with byproducts of combustion to allow conduct between the conductors of the sensor.
In the area of fire detection, one of the first popular fire detection systems relied upon heat sensors for providing an alarm of an occurring fire. Because the sensors were only responsive to high heat conditions, an alarm would be given only after the fire had entered its final stage, i.e. flame. In large warehouses or office buildings, this type of system at least had the advantage of preventing destruction of other parts of the building even though that part of the building in which the fire occurred was most likely destroyed. For residential application, however, to wait until a fire has entered the flame stage usually means substantial if not total destruction of the residence. Moreover, since the early stages of fire produce gases or byproducts of combustion, e.g. quite frequently carbon monoxide and hydrogen cyanide, the fumes from the fire would quite likely overcome the residents even though the fire has not entered the heat stage.
The smoke stage of the fire is, of course, particularly dangerous during nighttime hours when the inhabitants of the house are asleep. In order to provide an earlier warning of fires, smoke detectors, designed to provide an alarm during the early smoke stage of the fire, were developed. The two basic types of smoke detectors presently in use are the ionization type and the photoelectric type.
The ionization smoke detector normally involves a chamber having a first electrode, a second electrode and a radiation source positioned therebetween. The radiation source ionizes the air between the first and second electrodes to create an electric current between them which is detected by a detector circuit to maintain an alarm in an off condition. Should smoke enter the area between the first and second electrodes, called the sensing chamber, the smoke molecules which are larger than the ionized air molecules impede the progress of the ionized air molecules from one electrode to the other thus decreasing the current flow between these two electrodes. The detector detects the decreased current flow to sound the alarm. A second chamber, also having two electrodes and an ionization source therebetween, is sometimes provided in a sealed chamber so that it is responsive to changes in atmospheric pressure and humidity but not smoke. This second chamber, i.e. the reference chamber, compensates the detector for changes in humidity at atmospheric pressure.
The photoelectric type of detector has been developed in two configurations. The first is the light obscuration configuration in which a light source is mounted directly opposite a photocell such that, when no smoke is present, the light from the source impinges upon the photocell which is thereby made conductive. When smoke enters the sensing chamber, the light flowing from the source to the cell is obscured and the current flow through the photocell is reduced which is detected to provide an alarm. The second configuration is the light scattering type in which the light source and the photocell are mounted at right angles with respect to one another. With no smoke in the chamber, no light from the light source impinges upon the photocell and, therefore, the current flow through the photocell is small. As light enters the sensing chamber, lights reflects off of or is scattered by the smoke molecules and is detected by the photocell which results in an increased current. This increased current is detected to provide the appropriate alarm.